Compositions having selective electrical conductivity have been developed for various purposes. For example, U.S. Pat. No. 3,578,739 discloses that a composition having a resistance measured at 65.degree. C. at between 1.times.10.sup.6 and 1.times.10.sup.10 ohms is useful for a covered surface of a target electrode employed in an apparatus for electrostatically charging a continuous fibrous material (e.g., polyethylene) being forwarded in a linear path. Certain carbon-filled polymeric covering compositions for a metal electrode base are disclosed as suitable.
The electrostatic charging of flash-spun polyethylene may be accomplished by corona charging. Target electrodes used in these processes should have the ability to withstand corona potentials used in the charging process, should be suitably resistive to collect the corona current while inhibiting back-corona, should be structurally sound, and should be sufficiently hard to undergo mechanical cleaning. There is interest in developing ceramic materials for this application.
Historically, the invention of the transistor initiated the development of high purity single crystal semiconducting materials such as germanium and silicon. Today these materials are prepared with unparalleled perfection. In contrast, the development of electronic polycrystalline ceramics has been much more placid. The majority of polycrystalline electronic ceramics are produced using less defined synthesis and manufacturing processes. The resulting devices are often multicomponent and multiphase and contain significant microscopic and macroscopic defects. Undesirable impurity levels are orders of magnitude greater than in typical single crystalline semiconductors. In order to achieve polycrystalline perfection, processing and impurities levels must be controlled to avoid unwanted second phases, mixed valence conduction effects, and the segregation of impurities and second phase formations at grain boundaries.
The electrical conductivity of ceramic materials can encompass a wide range of values ranging from insulators to semiconductors to those of metallic conductors. Electrical conductivity in metals occurs by the movement of free electrons and is the result of bonding in the bulk crystal structure. In non-metallic materials such as ceramics, electrical conductivity can involve both electronic (migration of free electrons and holes) and ionic (migration of charged atoms) charge carriers and is the result of atomic imperfections (point defects) in the crystal structure or electronic imperfections (departure from stoichiometry). Atomic point defects are variations from the perfect periodicity of the crystal lattice. Four types of point defects are considered important in influencing conductivity: (1) vacancies (i.e., atomic sites in the crystal lattice which are not occupied that in the ideal crystal should be occupied); (2) interstitial atoms (i.e., atoms which occupy sites that in the ideal crystal should not be occupied); (3) misplaced atoms (i.e., atoms which occupy sites that in the ideal crystal are assigned to atoms of a different type; for example, in an AB crystal, a few A atoms may occupy B sites and vice versa); and (4) impurity atoms (i.e., impurity atoms which occupy normal host atom sites or interstitial host sties). Other defects that commonly occur in materials are dislocations, grain boundaries, and surface defects that interact with point defects to affect electrical conductivity. There can be considerable structural and compositional variations between the bulk crystal and regions close to dislocations, grain boundaries, and surfaces. These microscopic parameters often dominate electrical properties.
In polycrystalline materials (which by definition are comprised of individual crystallites separated by grain boundaries) defects, impurities and second phases are known to segregate to grain boundaries and strongly influence electrical properties (see, e.g., W. D. Kingery, J. Am. Ceram. Soc., 57 (1974) 1-8). It has been clearly established that grain boundaries either act as paths of high mobility to enhance electrical conductivity or inhibit transport with respect to the bulk and reduce conductivity (see, W. D. Kingery, Advances in Ceramics, Volume 1, (L. M. Levinson, Ed.) American Ceramic Society, Columbus, Ohio, (1981) 1-22). Since the tendency is for solutes to segregate to grain boundary regions, the atomic feature that most often has the greatest impact on electrical behavior in polycrystalline materials is the grain boundary region.
It is generally accepted that to enhance conductivity in polycrystalline materials, the level of impurity ions must be controlled and the formation of bulk and grain boundary second phases should be avoided. Silica is a particularly influential impurity since it is generally present as an impurity and has limited bulk solubility in ionic metal oxide ceramics. Silica segregates at grain boundaries forming amorphous films that often adversely affect electrical properties. Intrinsic grain boundary effects can, however, be used advantageously to create engineered electrical properties in polycrystalline systems that cannot be obtained in single crystal systems. Varistors, thermistors, and barrier layer capacitors are good examples of devices that exploit grain boundary effects to obtain their unique electrical properties.
Alumina has many desirable intrinsic physical properties including mechanical strength, temperature resistance, chemical inertness, and electrical resistance that are primarily determined by its crystal structure. The electrical resistivity of alumina-based ceramics is typically greater than about 10.sup.15 .OMEGA.cm at room temperature, which makes it suitable for use as insulators and electronic substrates. Indeed, chromia-alumina polycrystalline samples without magnesium cation additions are considered electrically insulating (i.e., their volume resistivity, .rho., is generally greater than about 10.sup.15 .OMEGA.cm) and magnesia-alumina polycrystalline samples without chromium cation additions are also considered electrically insulating (i.e., their .rho. is generally greater than about 10.sup.15 .OMEGA.cm). This is generally true at temperatures ranging from about 25.degree. C. to 600.degree. C.
Although alumina has desirable physical properties and is easily fabricated into functional shapes by standard ceramic forming and firing methods, its high electrical resistivity makes it unsuitable for applications requiring electrical conduction. Numerous ceramics are electrically conductive. However, most conductive ceramics have undesirable physical characteristics such as poor mechanical or thermal properties and are difficult to fabricate into useful shapes. A ceramic composition taking advantage of alumina's superior physical parameters, while being electrically conductive as well, would represent a particularly desirable ceramic.
With regard to electronic conductivity in oxides, there are generally two ways to enhance conductivity in the bulk crystal: by the departure from stoichiometry, or by the introduction of impurity atoms into a host lattice (controlled valence conduction). For alumina, conduction by departure from stoichiometry is unlikely because of alumina's high enthalpy of formation (1674.4 kJ/mol). Conduction from substitutional impurities is possible, however, though this has been shown to be limited at room temperature for alumina (see, F. A. Kroger, Advances in Ceramics, Volume 10, (W. D. Kingery, Ed.) American Ceramic Society, Columbus, Ohio, (1984) 1-15). Thus, any room temperature enhancement of electronic conductivity in alumina-based ceramics must come from grain boundary effects.
The majority of commercially available aluminas are sintered via a liquid phase route through the use of additives such as oxides of silicon, calcium, sodium, and potassium, often added in the form of minerals or clays. These additives enhance formation of siliceous liquid phases, and the presence of these viscous liquid phases during sintering aid densification at relatively low firing temperatures. They also form glassy (siliceous) grain boundary films on cooling. Even when liquid-forming additives are not used, sufficient impurities are generally present in the starting alumina powder to result in trace liquid formation upon sintering. Typical impurities include SiO.sub.2, CaO, Fe.sub.2 O.sub.3, TiO.sub.2, K.sub.2 O, and MgO. Siliceous grain boundary films can deleteriously affect properties such as thermal and electrical conductivity by scattering conducting photons or electrons. For electronic conducting materials it is desirable to eliminate glassy grain boundary films to minimize scattering effects. Thus, of primary importance in developing an electrically conductive alumina is the elimination of any siliceous grain boundary films and unintended second phases and unintended impurity ions that may segregate to grain boundaries.